Vasopressin elicits increased permeability to water and solutes in the apical plasma membrane of the epithelial cells which line the bladder of the toad, and similar changes in the epithelial cells of the medullary collecting duct of the mammalian kidney. Physiocochemical and biochemical techniques will be used to characterize the response of toad bladder epithelial cells to vasopressin. Changes in availability of proteins at the apical cell surface of the intact bladder in the presence and absence of hormone will be shown by labelling with iodine or phosphorus (by biochemical means) or with fluorescent probes of structure (by chemical techniques). Vasopressin effects on plasma membrane protein conformation will be studied by fluorescence spectroscopy in the intact bladder and in plasma membrane vesicles prepared from isolated epithelial cells, using extrinsic chromophores covalently bound to protein. Bladder epithelial cell populations will be separated on the basis of possession of hormone receptors and studied to determine whether the increases in water and solute transport occur in different cells and whether cell cooperation is required. The hormone-receptor interaction will be characterized in the intact cell. Effects of vasopressin on isolated plasma membrane fractions in vitro will be measured.